Warfarin (coumarin) is an anticoagulant which acts by inhibiting vitamin K -dependent coagulation factors. Warfarin based compounds are, typically, derivatives of 4-hydroxycoumarin, such as 3-(a-acetonylbenzyl)-4-hydroxycoumarin (COUMADIN). COUMADIN and other coumarin anticoagulants inhibit the synthesis of vitamin K dependent clotting factors, which include Factors II, VII, IX and X. Anticoagulant proteins C and S are also inhibited by warfarin anticoagulants. Warfarin is believed to interfere with clotting factor synthesis by inhibiting vitamin K1 epoxide regeneration.
An anticoagulation effect is generally seen about 24 hours after administration and single doses of warfarin are effective for 2 to 5 days. While anticoagulants have no direct effect on an established thrombus and do not reverse ischemic tissue damage, anticoagulant treatment is intended to prevent the extension of formed clots and/or to prevent secondary thromboembolic complications. These complications may result in serious and possibly fatal sequelae.
Warfarin is typically used for the treatment of in patients suffering from atrial fibrillation. Such treatment reduces the incidence of systemic thromboembolism and stroke. The FDA has approved warfarin for the following indications: 1) the treatment or prophylaxis of venous thrombosis and pulmonary embolism, 2) thromboembolic complications associated with atrial fibrillation and/or cardiac valve replacement, and 3) reducing the risk of death, recurring myocardial infarction, and stroke or systemic embolism after myocardial infarction.
A number of adverse effects are associated with the administration of warfarin. These include fatal or nonfatal hemorrhage from any tissue or organ and hemorrhagic complications such as paralysis. Other adverse effects include paresthesia, headache, chest abdomen, joint, muscle or other pain, dizziness, shortness of breath, difficult breathing or swallowing, unexplained swelling, weakness, hypotension, or unexplained shock. Other adverse reactions reported include hypersensitivity/allergic reactions, systemic cholesterol microembolization, purple toes syndrome, hepatitis, cholestatic hepatic injury, jaundice, elevated liver enzymes, vasculitis, edema, fever, rash, dermatitis, including bullous eruptions, urticaria, abdominal pain including cramping, flatulence/bloating, fatigue, lethargy, malaise, asthenia, nausea, vomiting, diarrhea, pain, headache, dizziness, taste perversion, pruritus, alopecia, cold intolerance, and paresthesia including feeling cold and chills.
Drug toxicity is an important consideration in the treatment of humans and animals. Toxic side effects resulting from the administration of drugs include a variety of conditions which range from low grade fever to death. Drug therapy is justified only when the benefits of the treatment protocol outweigh the potential risks associated with the treatment. The factors balanced by the practitioner include the qualitative and quantitative impact of the drug to be used as well as the resulting outcome if the drug is not provided to the individual. Other factors considered include the physical condition of the patient, the disease stage and its history of progression, and any known adverse effects associated with a drug.
It is important to note that drug toxicity is an important consideration in the treatment of individuals. Toxic side effects resulting from the administration of drugs include a variety of conditions which range from low grade fever to death. Drug therapy is justified only when the benefits of the treatment protocol outweigh the potential risks associated with the treatment. The factors balanced by the practitioner include the qualitative and quantitative impact of the drug to be used as well as the resulting outcome if the drug is not provided to the individual. Other factors considered include the clinical knowledge of the patient, the disease and its history of progression, and any known adverse effects associated with a drug.
Drug elimination is the result of metabolic activity upon the drug and the subsequent excretion of the drug from the body. Metabolic activity can take place within the vascular supply and/or within cellular compartments or organs. The liver is a principal site of drug metabolism. The metabolic process can be broken down into synthetic and nonsynthetic reactions. In nonsynthetic reactions, the drug is chemically altered by oxidation, reduction, hydrolysis, or any combination of the aforementioned processes. These processes are collectively referred to as Phase I reactions.
In Phase II reactions, also known as synthetic reactions or conjugations, the parent drug, or intermediate metabolites thereof, are combined with endogenous substrates to yield an addition or conjugation product. Metabolites formed in synthetic reactions are, typically, more polar and biologically inactive. As a result, these metabolites are more easily excreted via the kidneys (in urine) or the liver (in bile). Synthetic reactions include glucuronidation, amino acid conjugation, acetylation, sulfoconjugation, and methylation.
Drug therapy using warfarin is particularly difficult because the metabolism of warfarin is complex and subject to interactions with a host of other drugs, including drugs that are commonly prescribed in patients suffering from atrial fibrillation, such as amiodarone for example. Warfarin is a mixture of enantiomers having different intrinsic activities at the vitamin K epoxide reductase (VKER) enzyme. These enantiomers have different metabolic pathways using different CYP450 isozymes. The CYP450 metabolic system is highly inducible or repressible by a host of external factors such as diet and other medications. Also, the CYP450 system is subject to many genetic variations and has a low capacity and is easily saturable. For these reasons the metabolism of warfarin is subject to unpredictable variations and each enantiomer has a different metabolic fate and different potencies at the VKER enzyme.
In addition, warfarin activity at the VKER enzyme results in inhibition of coagulation factors II, VII, IX, and X, which have different half-lives of their own, ranging from hours (factor VII) to days (factor X). Because of this complex situation, the pharmacological effect (increased coagulation time) of warfarin becomes apparent only 5 to 10 days after a dose. It is therefore easy to understand why warfarin therapy is extremely difficult to predict and why patients are at high risk of bleeding complications including death. In the current state of warfarin therapy, patients on warfarin must report to a coagulation lab once a week in order to be monitored and in order to detect any early risk of bleeding complications. Even with this strict monitoring system, many patients on warfarin die every year from bleeding complications.
The potential clinical problems and business risk associated with developing drugs, which must past through the P450 metabolism “gauntlet”, is markedly increased in the United States by the following two facts: 1) the number of prescriptions filled in this country has increased to about 3 billion per year or 10 per person, and 2) patients, particularly those that live longer and have more complex medical problems, tend to take multiple medications. The latter issue is important because the incidence of ADRs rises exponentially when subjects take more than four drugs. Although it is good practice to avoid polypharmacy, in many cases this is not possible because patients require different classes of drugs to effectively treat complex medical conditions.
The landscape of drug R&D is littered by failed drugs that were withdrawn by the FDA because they caused fatal ADRs involving CYP metabolism. These drugs were clinically effective and in many cases commercially successful. Notable drugs that were withdrawn due to ADR-related deaths involving CYP450 metabolism include terfenadine (February 1998), astemizole (July 1999) and cisapride (January 2000). In each of these cases, drug interactions involving CYP3A4 caused concentrations of the pharmaceutical agent to increase to such a degree that it significantly inhibited a particular type of potassium channel in the heart called IKr, which in turn, prolonged the QT interval and caused a potentially fatal form of ventricular tachyarrhythmia called torsades de pointes.
A warfarin analog that has a controllable and a predictable metabolic fate, not depending on CYP450, is therefore highly desirable and would be an important addition to the armamentarium of drugs available for treating atrial fibrillation patients.